Four things to consider in reading a tree for felling. Lean, branch size, balance and windage. Removed limbs don't catch moving air, large branches don't roll the log when they fail to snap if they're already on the ground. As a side note, people underestimate the amount of sail area a tree can represent. My Uncle was a topper (for skyline yarding operations - Oregon) and his best story was about the windless day and the tree top that wouldn't fall. He got madly to work with the wedges before it decided on a random direction involving him when the hinge gave way.
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Fiasco LabsFeb 24 '13 at 5:21